The Invention relates to recovery of oil and gas from wells, and more particularly to decreasing fluid loss and to breaking fluids inside formation pores when using viscoelastic surfactant fluid systems (VES's) as carrier fluids and treatment fluids.
There are many oilfield applications in which filter cakes are needed in the wellbore, in the near-wellbore region or in one or more strata of the formation. Such applications are those in which, without a filter cake, fluid would leak off into porous rock at an undesirable rate during a well treatment. Such treatments include drilling, drill-in, completion, stimulation (for example, hydraulic fracturing or matrix dissolution), sand control (for example gravel packing, frac-packing, and sand consolidation), diversion, scale control, water control, and others. Typically, after these treatments have been completed the continued presence of the filter cake is undesirable or unacceptable. In such oilfield operations as hydraulic fracturing and gravel packing, viscoelastic surfactant (VES) fluid systems are popular as carrier fluids because of their ability to create a very clean proppant or gravel pack. However, they sometimes experience undesirably high fluid loss, especially when formations with permeabilities greater than about 5 mD are treated. Consequently, fluid loss additives (FLA's) are often used with such carrier fluids to reduce leak off.
There are also many applications in which breakers are needed to decrease the viscosity of treatment fluids, such as fracturing, gravel packing, and acidizing fluids. Most commonly, these breakers act in fluids that are in wellbores or fractures; some breakers can work in fluids in formation pores. Breakers decrease viscosity by degrading polymers or crosslinks when the viscosifiers are polymers or crosslinked polymers. Breakers decrease viscosity by degrading surfactants or changing or destroying micellar structure when viscosifiers are viscoelastic surfactant fluid systems.
Solid, insoluble materials, such as mica, (that may be called fluid loss additives (FLA's), lost circulation additives, and filter cake components) are typically added to fluids used in certain treatments to form filter cakes when they are needed, although sometimes soluble (or at least highly dispersed) components of the treatment fluids themselves (such as polymers or crosslinked polymers) may form the filter cakes, provided that the polymers or crosslinked polymers are too large, or rock pores are too small, to permit entry of much of the polymer or crosslinked polymer. This filter cake is typically on a surface, such as a fracture face. Removal of the filter cake is typically accomplished either by mechanical means (scraping, jetting, or the like), by subsequent addition of a fluid containing an agent (such as an acid, a base, or an enzyme) that dissolves at least a portion of the filter cake, or by manipulation of the physical state of the filter cake (by emulsion inversion, for example). These removal methods usually require a tool or addition of another fluid (for example to change the pH or to add a chemical). This can sometimes be done in the wellbore but normally cannot be done in a proppant or gravel pack. Sometimes the operator may rely on the flow of produced fluids (which will be in the opposite direction from the flow of the fluid when the filter cake was laid down) to loosen the filter cake or to dissolve the filter cake (for example if it is a soluble salt). However, these methods require fluid flow and often result in slow or incomplete filter cake removal. Sometimes a breaker may be incorporated in the filter cake but these must normally be delayed (for example by esterification or encapsulation) and they are often expensive and/or difficult to place and/or difficult to trigger.
There would sometimes be advantages to forming a filter cake inside the pores of a formation. For example, such an “internal” filter cake would not be subject to erosion by fluids flowing across a filter cake that was formed on a wellbore face, a screen, a fracture face, or similar location. Also, an internal filter cake could be more effective at reducing “spurt” the initial fluid loss that occurs as a filter cake is being formed. However, formation of internal filter cakes is usually avoided, since in the past they have been difficult, if not impossible, to remove.
There is sometimes a need to break viscous fluids within the pores of formations, for example when viscous fluids enter formations during fracturing, gravel packing, acidizing, lost circulation treatments, scale squeezes, and the like. These fluids that enter the formation may be main treatment fluids (such as fracturing fluids) or they may be secondary fluids (such as diversion fluids or viscoelastic diverting acids). Most breakers are solids, for example granules or encapsulated materials, that do not enter the formation.
The use of a hydrolysable polyester material for use as an FLA for conventional fluid loss control has previously been proposed; further, degradation products of such materials have been shown to cause delayed breaking of fracturing fluids. U.S. Pat. No. 4,715,967 discloses the use of polyglycolic acid (PGA) as a fluid loss additive to temporarily reduce the permeability of a formation. SPE paper 18211 discloses the use of polyglycolic acid (PGA) as a fluid loss additive and gel breaker for crosslinked hydroxypropyl guar fluids. U.S. Pat. No. 6,509,301 describes the use of acid forming compounds such as PGA as delayed breakers of surfactant-based vesicle fluids, such as those formed from the zwitterionic material lecithin. The preferred pH of the fluid in which these materials are used is above 6.5, more preferably between 7.5 and 9.5. The use of such materials as destroyable internal filter cakes is also known. U.S. Patent Application Publication No. 2005-0252659, assigned to the assignee of the present application and hereby incorporated in its entirety, describes a method in which the fluid contains a solid hydrolysable polyacid that upon dissolution and hydrolysis releases an acid that is a breaker for the viscosifying system. Suitable solid hydrolysable polyacids include polylactic acid and polyglycolic acid. The fluid in that patent application also contains a pH control agent, present in an amount sufficient to neutralize any acid present in the solid hydrolysable polyacid before the injection and to neutralize any acid generated by the solid hydrolysable polyacid during the injection, so that the acid breaker is not available to break the fluid during the injection. In one embodiment the viscosifier is a viscoelastic surfactant fluid system and the solid hydrolysable polyacid is of a size selected to be a fluid loss additive, for example in fracturing or gravel packing. In another embodiment, the solid hydrolysable polyacid is used in particles sufficiently small that they enter the pores of the formation. In either case, the viscosifier is broken after the solid releases more acid than can be neutralized by the pH control agent. U.S. Patent Application Publication No. 2006-0157248, assigned to the assignee of the present application and hereby incorporated in its entirety, describes an oilfield treatment in which particles of a polyol such as polyvinyl alcohol are injected into a well. The particles are initially insoluble and are made soluble by heating or by a decrease in salinity. The particles may form an internal filter cake, and after solubilization, they may be breakers.
Additional compositions and treatment methods in which an internal filter cake is formed, and then the filter cake and the viscosifiers used in the treatment are destroyed, would be of value. It would be desirable to have a number of materials that are available in small particles that could be used in a treatment fluid to form an internal filter cake in a formation and that then would decompose without mechanical or chemical action by the operator so that the filter cake was destroyed, releasing degradation products that break viscoelastic surfactant-based viscosifiers in the fluid. It would also be desirable if such materials could be used in particles so small that they did not form a filter cake but still acted as breakers. It would be desirable to have a number of such materials so that they could be used under different subterranean conditions, for example temperature and formation fluid chemistry.